Don’t be a …

今天在BT群由pMq发了一张图，并问是什么意思：

这个问题的答案是……不那么……显而易见的。

任何具有基础物理知识的人，都会知道：

1. 一般而言，t代表时间。
2. 一般而言，特别是牵涉到对时间求导时，x代表了位移（displacement）。
3. 位移对时间的一次求导，代表了速度v（velocity）。
4. 位移对时间的二次求导，也就是速度对时间的一次求导，代表了加速度a（acceleration）。

一般而言，到此为止了。我们很少会关心位移对时间更高次的求导。

=== 以下是科普时间 ===

这里的三次求导结果，和速度、加速度一样，在物理上是有一个明确的定义的，其专用术语是jerk。但……尴尬之处在于，jerk本身有不少很糟糕也更popular的定义。

按照维基百科的词条，jerk作为名词，有7个释义，列举如下：

1. A sudden, often uncontrolled movement, especially of the body.
2. A quick, often unpleasant tug or shake.
3. (Canada, US, slang, derogatory) A person with unlikable or obnoxious qualities and behavior, typically mean, self-centered or disagreeable.
   1. I finally fired him, because he was being a real jerk to his customers, even to some of the staff.
   2. You really are a jerk sometimes.
4. (US, slang, derogatory) A dull or stupid person.
5. (physics, engineering) The rate of change in acceleration with respect to time.
6. (obsolete) A soda jerk.
7. (weightlifting) A lift in which the weight is taken with a quick motion from shoulder height to a position above the head with arms fully extended and held there for a brief time.

请特别注意3/4/5/7这四个定义。

首先，jerk是个举重专用术语，表示“挺举”。对应的，“抓举”的英文是snatch。亚运会马上就要开了，大家可以关注一下举重比赛，中国在举重上实力也很强的。

其次，在美加的语境下，jerk基本就是一个贬义词，根据语境，可以翻译为“祸害”、“猪头”、“烂人”等等。

第三，也就是文章开头图片提到的，位移对时间求三次导数，得到的是加速度的变化——我乐意称为“加加速度”（jerk）。

所以，上图是说：

Don’t be a JERK!

=== 以下是不正经笑话时间 ===

在英语中有众多关于jerk的笑话；当然，关于blondie（金发女郎）的笑话就更多了。这些笑话基本都不正经，所以为了爱护花花草草和小朋友，我就不列举这些笑话了。

不过如此一来，众位看官肯定不开心，说不定会循着网线来找我当面complain。

那么还是放两个可以放的jerk的笑话吧。

其一：

"What’s your name, son?" The principal asked his student.
The kid replied, "D-d-d-dav-dav-david, sir."
"Do you have a stutter?" the principal asked.
The student answered, "No sir, my dad has a stutter but the guy who registered my name was a real jerk."

其二：

One day, inexplicably, my talking parrot started insulting me.
He called me an idiot, a fool, a jerk, stupid, and a variety of other nasty names.
I warned the squawker to cease, but to no avian avail.
Fed up, I finally flipped the foul-mouthed feather-brain into the freezer…but after about 15 seconds, I relented and let him out.
"I’m so sorry," he declared! "I don’t know what came over me, and realize I shouldn’t have said those terrible things. I hope you can forgive me, and I promise never to do it again! By the way…what did the chicken do?"

就到这里！就到这里！休息！